To provide a locking device in which a door can be opened even with a lock piece operating member inadvertently removed with leaving a lock piece at a lock position.

The locking device include a latch bolt 4 freely protruding in an advancing/retreating manner from the end face of the free end side of the door 2, and a lock piece 6 freely changeable in position between a lock position regulating the retreat of the latch bolt 4 and a lock releasing position permitting the retreat of the latch bolt 4, wherein the lock piece 6 is operated to change the position with a lock piece operation member 29 between the locking position and the lock releasing position. When the lock piece operation member 29 is removed while the lock piece 6 is operated at the lock position with the lock piece operation member 29, the lock piece 6 is configured to return to the lock releasing position from the lock position.